Heat Management, or "How Do I Not Burn to Death?"

[RocketBlam]

Friction and heat have gotten a lot more unforgiving with the more recent versions of the game, so here are a few ideas to help you reenter an atmosphere without turning into a long streak of glowing ash.

First, remember that different planets have vastly different atmospheres. You can re-enter Kerbin from an orbit of 80,000 meters by setting your periapsis at 30k, and you should be fine. However, if you try to do that on Eve, all you will remember is a flash of red and searing pain. In order of atmospheric thickness (roughly), it goes like this, from thinnest to thickest.

The Planets

1. Duna. The air here is so thin that aerobraking is almost a waste of time. You will often not even be able to use parachutes, as you will impact the ground at high speed before you can deploy them. The atmosphere extends to 50,000 meters.

2. Laythe. The atmosphere here is very similar to Kerbin, although a bit thinner. Aerobraking and flying are definitely feasible here. The atmosphere extends to 50,000 meters, like Duna, but is much thicker.

3. Kerbin. Medium thickness. You should be pretty familiar with it. De-orbiting by setting your periapsis at 30k, from an orbit of 75k, should work fine. Coming in from Duna or Eve, you won't want to try anything below 60k. The atmosphere tops out at 70,000 meters.

4. Eve. Now we're getting serious. You have to be careful aerobraking here. If you're coming in from Kerbin, setting your periapsis below 80k will probably get you killed. The last time I aerobraked here, coming in from Kerbin and setting my periapsis at 80k, all of my landing struts evaporated within 10 seconds. You're best bet is to use a heat shield that covers the entire craft (not just the bottom). That is, for a 2.5m craft, you may want to use a 3.75m heat shield, if you have things like landing struts sticking out. Even from an orbit of 100k (10k above the edge of the atmosphere) you're not going to want to go below 70k, as the atmosphere gets very thick, very quick. Hey, you see what I did there? The atmosphere extends to 90,000 meters.

5. Jool. I wouldn't even try it. The last time I tried to aerobrake in Jool's atmosphere, I was vaporized no matter how high I tried to set my periapsis. This was in 1.04 though, things might have changed. That whole planet says "Go Away". The atmosphere extends to a whopping 200,000 meters.

Technique

Here are a few ways to manage heat load, beyond the obvious, like using heat shields.

1. Spaceplanes

There are two good ways to reduce heat load in spaceplanes.

A. Angle. The first is to angle your vessel up, so that it is not diving straight into the atmosphere. This increases drag, and may also allow you to skip off of the atmosphere to some degree. If you set the angle high enough (say, 45 degrees above prograde, at 60k on Kerbin) you may even start gaining altitude again without engines. As you get lower, the angle has to go down, or you risk having your wings fly off. So at 30k on Kerbin, I would use an angle more like 20 degrees.

Angling reduces heat load, and may help you slow down if you are low enough. At lower altitudes it becomes a tradeoff between slowing down and not overheating. High friction means high drag, but strangely, lower heat.

It goes like this: If you point straight toward prograde, you will slow down, but you will lose altitude quickly, and gain heat quickly. And of course the more altitude you lose, the thicker the air, so the greater the drag and the heat. If you angle up, you will lose altitude more slowly (or even gain altitude), and gain heat much more slowly.

This just takes practice, to know when to pitch up and relieve the heat load and when to pitch back down to get more braking effect. Incidentally, the early Mercury/Gemini astronauts did exactly this to relieve heat load. 

B. Air brakes. These are moderately effective at slowing your craft down. They work best at lower altitudes and higher atmospheric densities. When using these, you can angle lower, pointing your ship more toward the front. A combination of air brakes and angling will give you the most options for dealing with heat.

2. Not-Spaceplanes

You can't normally angle in a lumpy spacecraft without wings. In fact it doing so will probably start destroying things on the sides and top of your spacecraft. There is one technique I've discovered that helps, though: Spinning. That is, rotating around your roll axis. The default keys for this are Q and E. I've found that if you have things that are getting close to overheating, and especially if they are mounted on the sides of your ship and thus exposed to friction, if you start rolling the ship, the heat load is dissipated almost immediately. This probably won't work for things on the very bottom of your ship like your engine (assuming you are flying in retrograde), because spinning doesn't really change the angle that they fly into the atmosphere. However, I usually have something or other sticking out the side of the craft (radial mount parachutes, solar panels, Goo containers) that start overheating because they are exposed to the friction of the atmosphere. For whatever reason, spinning the craft (that is, rolling) usually relieves the heat load quickly. Unfortunately, once you stop spinning the heat load builds up again, but if you just keep spinning like that, although any passengers will probably eject their lunch, the craft should weather the friction until you can get down to a slower, safer speed.

I hope this helps. Stay cool! Hehe... see, I said... cool... never mind.

Edited December 16, 2015 by RocketBlam

[Guest]

Jool has changed. I had no problem with aerocapture at Jool, was quite easy in fact. The aggravating thing is once you get below 1,000km and stop the time warp you cannot timewarp again until you go back over 1,000km. Keep in mind, I used a basic rocket with mk1 lander can, Rockomax X200-32 large fuel tank, and lv-909. The large fuel tanks create quite a bit of drag, so when coming in retrograde with less than half a tank of fuel, they make semi-good heat shields and slow down the craft enough for aerocapture.

Spaceplanes can aerocapture at Duna quite easily with high AoA, as high as you can go and still maintain control( which will be different for different planes). Same goes for Kerbin return, it's just a little more hairy, with the altitude window needing to be much more precise. Plane may blow up at 37km, have successful aerocapture at 39km, and still need engines to finish the job at 41 km(not exact figures and will depend on a number of factors, just an example). As far as Eve goes, yeah, good luck. Have not tried Jool system missions with spaceplanes yet, only rockets.

[RocketBlam]

I think I heard during 1.04 that they were going to fix it. I mean, it was bad. You couldn't touch Jool's atmosphere without almost immediately exploding. Prior to 1.04, aerobraking was no big deal at Jool, I did it several times. So, it looks like they fixed it, which is good news.

I haven't taken a spaceplane to Duna in quite some time, so my comments were for just regular, wingless craft. But it is kind of strange... a couple of missions ago, I aerobraked and used chutes on my first try on Duna. When I went back with a manned vessel, I had to try about 5 times before I could even slow down enough to use chutes safely, and that was only by thrusting to slow down.

Edited December 16, 2015 by RocketBlam

[superstrijder15]

Spinning end over end also helps to dissapate heat, I think this is because it's impossible to point perfectly retrograde, and thus one side will be more exposed if you try, that would also explain roll. Bonus is extra drag thus slowing more, I made this technique while trying to aerobrake from mun without burning and using my lander as a good heatshield.